This invention relates to methods for gasifying subterranean carbonaceous deposits and more particularly to methods for gasifying such deposits with the aid of specific patterns of injection and production well bores.
One prior art process for gasifying subterranean carbonaceous deposits such as coal over a wide area is known as the "line drive process". In that process, well bores are drilled at suitable spacing in parallel rows extending generally perpendicular to the direction of the line drive, i.e., the direction in which the gasification or burn front is to progress. After a combustion link is established between two adjacent rows of well bores by well-known means, a free oxygen containing gas is injected into the well bores of one such row (the injection wells) with ignition and gasification of the carbonaceous deposit being accomplished between the two rows, gasification product gas being recovered from the well bores of the second of the two rows (the production wells).
When it is desired to continue the gasification of the deposit in the same line drive direction, the production wells are converted to injection wells with gasification occurring between such wells and a succeeding adjacent row of well bores which now become the production wells for the next gasification pattern of the line drive.
One drawback with a line drive pattern of this type is the use of well bores for both injection and production, which results in less efficient gasification of the subterranean deposit than could be accomplished if the wells were used for a single purpose, either injection or production. The reason is that when linking is attempted from a well which has been previously used for either injection or production, it is difficult to control the position of the combustion link in the deposit so that combustion can be initiated near the bottom of the subterranean deposit, which is known to be a desirable objective in this art.
Another problem inherent in any line drive process of the type described is the problem of water influx into the gasification zone from the surrounding areas. Since coal seams tend to act as aquifers, natural water will seek to travel through a coal seam except when interrupted, such as by sufficiently high pressures accompanying gasification. However, some encroachment of such water into any gasification pattern is inevitable. The greater the length of the outer perimeter of the pattern undergoing gasification at any given time in relation to its cross-sectional area, the greater the amount of such water influx which may interfere with combustion efficiency. Since the conventional short-spacing line-drive pattern described above extends at any given time only the distance between two adjacent rows of well bores, such a pattern possesses a relatively long length of exposed perimeter in relation to the area undergoing gasification.
A further problem with the aforementioned conventional line drive is that each succeeding gasification pattern in the direction of the line drive is necessarily in direct contact with a previously burned-out pattern. As a result, gasification products generated in the succeeding pattern may escape to the surface through such burned-out zone, with consequent environmental pollution and personnel hazard.
It is therefore a general object of this invention to provide an improved process whereby to gasify in situ a subterranean carbonaceous deposit.
It is a more specific object of this invention to provide such a process wherein a large area of such carbonaceous deposit may be more efficiently produced.
It is a still further object of this invention to provide a process of in situ gasification of such a carbonaceous deposit in which the problem of water influx is minimized.
Other and further objects and advantages of this invention will become apparent from consideration of the detailed description to follow taken in conjunction with the drawings and the claims appended hereto.